SiC composites

Accepted Manuscript Title: Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf /SiC composites Authors: Tao Han, Ruiying Luo, Guangyuan Cui, Lianyi Wang PII: DOI: Reference:

S0955-2219(19)30018-4 https://doi.org/10.1016/j.jeurceramsoc.2019.01.018 JECS 12285

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

18 September 2018 7 January 2019 10 January 2019

Please cite this article as: Han T, Luo R, Cui G, Wang L, Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf /SiC composites, Journal of the European Ceramic Society (2019), https://doi.org/10.1016/j.jeurceramsoc.2019.01.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf/SiC composites Tao Hana, Ruiying Luoa,b,*, Guangyuan Cuia, Lianyi Wanga a

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School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China b School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China E-mail address: [email protected]

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Abstract SiC-nanowire-reinforced SiCf/SiC composites were successfully fabricated through an in situ growth of SiC nanowires on SiC fibres via chemical vapour infiltration. The dielectric and microwave absorption properties of the composites were investigated within the frequency range of 8.2–12.4 GHz at 25–600 °C. The electric conductivity and complex permittivity of the composites displayed evident temperature-dependent behaviour and were enhanced with increasing temperature. The composites exhibited superior microwave absorption abilities with a minimum reflection loss value of −47.5 dB at 11.4 GHz and an effective bandwidth of 2.8 GHz at 600 °C. Apart from the contribution of the interconnected SiC nanowire network and multiple reflections, the excellent microwave absorption performance was attributed to dielectric loss that originated from SiC nanowires with abundant stacking faults and heterostructure interfaces. Results suggested that the composites are promising candidates for high-temperature microwave absorbing materials.

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Keywords: SiC nanowires, SiCf/SiC composites, high temperature, electrical conductivity, microwave absorption

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1. Introduction The rapid development of wireless communication technology and electronics industries has led to the widespread use of electronic devices in civil and military fields [1–7]. However, electromagnetic (EM) radiation generated from electronic devices not only causes severe EM interference and information leakage but is also harmful to biological systems[8,9]. Thus, the rising challenge is the development of lightweight materials with high microwave attenuation that satisfies the requirements of modern EM absorption applications and aids the EM purification of the environment. In the past decades, considerable efforts have been directed toward exploring high-performance microwave absorption materials, such as carbon materials and ferromagnetic metals [10–12]. However, these

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candidates for microwave absorption materials are restricted only to lowtemperature environments because carbon materials are susceptible to oxidation above 400 °C and ferromagnetic materials tend to undergo ferromagnetic–paramagnetic transition at the Curie point, resulting in the loss of absorption properties[13–16]. Hence, effective microwave absorption materials must satisfy the requirements of excellent mechanical and microwave absorption properties at high temperatures for applications such as aerospace industry, especially for the high-temperature engine components. Compared with conventional materials, ceramic matrix composites, which have a combination of functional and structural materials, exhibit better properties and wider application prospects as high-temperature absorption materials [17–21]. SiCf/SiC composites are among the most notable ceramic composites that can be utilised in high-temperature and harsh environments. These composites possess satisfactory hightemperature strength, thermal–chemical stability, and lightweight characteristics, rendering them promising candidates for high-temperature microwave absorption materials [22–26]. However, these composites cannot achieve a high absorption performance because of their single polarisation mechanism, low conductivity and relative dielectric permittivity. To address this problem, SiCf/SiC and dielectric materials can be combined to improve the absorption performance by introducing multiple polarisations along with the relaxation behaviour and adjusting the dielectric properties. Among different types of dielectric materials, SiC nanowires have elicited considerable attention because of their unique properties, such as high thermal stability, chemical resistivity and superior mechanical strength. In addition, SiC nanowires exhibit good absorption performance because of their high specific surface areas, abundant stacking faults and adjustable electrical conductivity[27–29]. Owing to the excellent properties of SiC nanowires, incorporating them into SiCf/SiC composites can effectively improve conductivity and dielectric loss, thereby enhancing the microwave absorption properties. However, limited studies have been conducted yet on the high-temperature dielectric and microwave absorption characteristics of SiCf/SiC composites with SiC nanowires, despite that their promising high-temperature microwave absorption properties have elicited increasing attention. Therefore, the dielectric mechanism and high-temperature absorption behaviour of SiCf/SiC composites with SiC nanowires should be comprehensively investigated. In this study, SiC nanowires were first introduced into SiCf/SiC composites through chemical vapour infiltration (CVI) to optimise the EM absorption performance. The dielectric and microwave absorption properties of the composites at 25–600 °C in the X-band (8.2–12.4 GHz)

were studied, and possible absorption mechanisms were proposed. 2. Experimental details

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2.1 Materials The polymer-derived KD-II SiC fibre was provided by the National University of Defense Technology, China. The preforms used as reinforcements in this work were three-dimensional four-directional braided SiC fibre fabric. The fibre volume fraction of the preforms was controlled at approximately 50%. Polycarbosilane (PCS) with an average molecular weight of 1250 was used as the precursor of the matrix provided by National University of Defense Technology, China. Methyltrichlorosilane (MTS) with a purity of 99% was supplied by Aldrich Chemical Co., Ltd. and served as the Si and C sources.

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2.2 Preparation of the SiC nanowires The SiC nanowires were grown on the fibre surface by CVI. Prior to the in situ growth of the SiC nanowires, the preforms were treated under a N2 atmosphere at 700 °C for 3 h to remove the agglutinant over the fibres for the subsequent use. Then, the preforms were hung in the CVI furnace. The furnace was heated up to 1100 °C from room temperature at a rate of 4 °C/min and maintained for 2 h. The SiC nanowires were grown directly on the fibre surface by introducing MTS as a source precursor into the hot chamber. H2 was used as both the carrier and diluent gas at flow rates of 150 and 1000 mL/min, respectively. The flow rate of MTS was controlled by that of the carrier gas, and the volume ratio of H2 to MTS was 120. Argon gas was steadily supplied at a rate of 800 mL/min during the growing process to maintain an inset atmosphere. During the growth process, the pressure in the furnace was monitored and maintained at 1.47 kPa by using a throttle valve. In this way, the preforms with in situ grown SiC nanowires (SiCnw-SiC) were obtained.

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2.3 Preparation of composites The as-received SiCnw-SiC preforms were subjected to a densification process via precursor infiltration and pyrolysis (PIP). First, these preforms were infiltrated into the 50% PCS/xylene solution as a precursor of the SiC matrix and dried at 80 °C for 5 h to remove the xylene. Then, the preforms with PCS were treated at 900 °C with N2 as the protective atmosphere. Finally, 10 cycles of the PIP treatment were performed to obtain the final specimens until the weight gain was less than 1 wt%. Thus, SiCf/SiC composites with SiC nanowires were fabricated and referred to as the SiCf/SiC-SiCnw composites. For comparison, SiC preforms without in situ grown SiC nanowires were prepared via the same PIP process under the

same conditions and were referred to as the SiCf/SiC composites.

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2.4 Characterisation The porosity and density of the composites were measured using the Archimedes method. The flexural strength and elastic modulus of the composites were evaluated by performing three-point bending tests on an Instron 5565-5kN system. The morphology and microstructure of the nanowires grown on the SiC fabric were characterised using a field emission scanning electron microscope (SEM) (Model S-4800, Japan) equipped with an energy dispersive spectroscope (EDS) and a highresolution transmission electron microscope (HRTEM, Model JEM-2100; Japan), along with selected area electron diffraction (SAED) operated at 200 kV. The X-ray diffraction (XRD) patterns examined by using Model D/max 2200 (Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å) were used to analyse the crystalline structures of the samples. The range of 2θ values recorded in the XRD patterns was 20°–80° with a scanning speed of 4° min−1 at room temperature. The Raman spectra were obtained using a Renishaw Ramoscope (Confocal Raman Microscope; Invia; Renishaw, Gloucestershire, UK) with a He−Ne laser wavelength of 514 nm. The volume electrical conductivity of the samples was measured according a two-probe method by using Keithley 6220 DC Precision Current Source (Tektronix Instruments Inc., Cleveland, USA). The complex permittivity and scattering parameters (i.e. S-parameters: S11, S12, S21 and S22) of the specimens with dimensions of 22.86 mm × 10.16 mm × 2 mm were measured through a waveguide method by using a twoport vector network analyser (MS4644A; Anritsu, Kanagawa, Japan) in the frequency range of 8.2–12.4 GHz at four temperature spots, namely, 25, 200, 400 and 600 °C. The high-temperature waveguide measurement setup is schematically shown in Fig. 1. In the measurement process, the samples were placed perpendicularly at the centre of the test chamber and heated by a heater at 10 °C/min. To ensure high accuracy of each measurement at a specific temperature, the sample was retained in each temperature spot for 15 min to stabilise the testing system prior to the measurement. To eliminate artificial measurement errors and minimise the effect of air gaps between the sample and the holder, three specimens of composites were fabricated for the test. 3. Results and discussion 3.1 Morphology of SiC nanowires Fig. 2 shows the SEM micrographs and corresponding EDS spectra of the pristine SiC and SiC fibres with SiC nanowires coated on the surface. As shown in Fig. 2(a), the surface of the SiC fibre was smooth, and its

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diameter was approximately 13 μm. Fig. 2(b) shows the SEM image of the overall appearance of the SiC nanowires grown on the SiC fibre at a low magnification. Well-distributed, wire-like, one-dimensional structures were formed and entirely covered the SiC fibres. The nanowires with lengths of approximately tens of micrometers had a large aspect ratio. The magnified SEM images of the SiC nanowires are shown in Fig. 2(c). Both curvy and straight nanowires with random orientations can be observed. The structure of curvy nanowires was due to the orientation change of the growth direction caused by the weak airflow effect force and other attractive/repulsive or stimulating forces attributed to a slight pressure or temperature difference[30]. The curvy or straight nanowires interconnected with one another to form a network structure, which promoted the attenuation and absorption of microwave energy. The local magnified image embedded in Fig. 2(c) shows that the SiC nanowires were cylinder-shaped with very smooth and clean surfaces and had diameters of approximately 100 nm. The EDS analysis corresponding to the selected area in Fig. 2(c) revealed that the main chemical elements in the sample were Si and C, suggesting that the nanostructures were mainly composed of SiC. A TEM was employed to further analyse the internal structure of the as-grown SiC nanowires. Figs. 3(a) and 3(b) show the typical and highmagnification TEM images of the SiC nanowire grown on the surface of SiC fibre. As shown in Fig. 3(a), the SiC nanowire has a smooth surface along the length and a diameter of approximately 100 nm, which was consistent with the obtained SEM results. Fig. 3(b) shows numerous steplike streaked lines in the crystal plane perpendicular to the axis of the nanowires, which were considered to be high-density stacking faults. This phenomenon was attributed to the embedding of 2H-SiC segments into the 3C-SiC[31]. Fig. 3(c) shows the HRTEM image of the area marked with white square in Fig. 3(b). The interplanar spacing was measured as 0.25 nm, which corresponded to the spacing of the {111} lattice planes of β-SiC, thus implying that the nanowires grew along the [111] direction. Moreover, a SiO2 amorphous layer with a thickness of approximately 5 nm can be observed on the nanowire surface, resulting from the SiC oxidation or SiO conversion during preparation [32]. The inset in Fig. 3(c) shows the SAED patterns of the SiC nanowire. It depicts the existence of streaks and diffraction spots, which can be indexed on the basis of the 3C-SiC crystal, indicating that a SiC nanowire with high-density stacking faults was produced.

3.2 Phase composition of SiC nanowires To assess the crystalline structures of the as-received materials, the fibre fabric covered with nanowires was analysed on the basis of the XRD

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patterns and Raman spectra. Fig. 4(a) shows the XRD patterns of the SiC fabric with in situ grown SiC nanowires and those of the SiC fibre fabric for comparison. No evident XRD peaks, except for a discernible broad band at 2θ = 35.3°, can be observed in the pattern of the SiC fibre fabric, implying that the SiC fibre had an amorphous structure. Through comparison, the XRD patterns of the SiC fabric with in situ grown SiC nanowires exhibited several strong peaks at approximately 35.5°, 41.3°, 60.3°, 71.6° and 75.4°, which corresponded to (111), (200), (220), (311) and (222) crystal faces and agreed with those of the β-SiC. A small shoulder with low intensity was observed at 2θ = 34° near the strong (111) peak and was ascribed to the stacking faults that spontaneously formed during the growth of the nanowires[33,34]. The results verified the existence of SiC nanowires in the fabric. Fig. 4(b) shows the Raman spectra of the SiC fabric with in situ grown SiC nanowires and the SiC fibre fabric. The D peaks at 1350 cm−1, which was attributed to the defects and disorder degree, and the G peaks at 1600 cm−1, which was associated with sp2 hybridized carbon bonds, can be observed in both spectra [35,36], suggesting that both fabrics have abundant carbon. Compared with the SiC fibre fabric, two additional peaks appeared in the SiC fabric with SiC nanowires spectrum. A strong peak appeared at 785 cm−1 and was associated with the mode of the transverse optical (TO) phonons of SiC. Moreover, a small peak appeared at 940 cm−1, which corresponded to the SiC longitudinal optical (LO) mode. The significant difference in height between the TO and LO phonon peaks indicated the SiC growth along a single direction by selection rules for crystal structures [37]. In addition, the two peaks in our sample slightly shifted by 11 and 23 cm−1, respectively, relative to the standard values of the single crystal 3C-SiC located at 796 and 963 cm−1, respectively. This phenomenon was ascribed to the stacking faults and inner stress in SiC nanowires [38]. These findings confirmed that SiC nanowires with stacking faults were synthesised successfully, which was consistent with the XRD results.

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3.3 Mechanical properties of the composites Table 1 lists the physical properties of the fabricated SiCf/SiC-SiCnw and SiCf/SiC composites for comparison. The SiCf/SiC composites had a density and porosity of 2.10 g/cm3 and 7.73%, respectively. In comparison, the SiCf/SiC-SiCnw composites showed a higher density of 2.21 g/cm3 and a lower porosity of 5.67%. The nanowires that were uniformly grown between the pores and gaps in the inter- and intra-bundles of fibres provided the appropriate positions for the matrix infiltration. Such a structure enhanced the efficiency of infiltration and pyrolysis, resulting in SiCf/SiC-SiCnw with a higher density and lower porosity. The flexural

strength of SiCf/SiC-SiCnw increased by 82.3% to 613.8 MPa compared with that of SiCf/SiC (336.7 MPa). The elastic modulus of SiCf/SiC-SiCnw (86.8 GPa) was higher than that of SiCf/SiC (53.3 GPa), indicating that the in situ grown SiC nanowires can significantly improve the mechanical properties of the composites.

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3.4 Electrical conductivity of the SiCf/SiC composites For SiCf/SiC composites as non-magnetic materials, the electrical conductivity is critical in the microwave absorption property of the materials. Fig. 5 shows the variation in electrical conductivities of the two composites at various temperatures. The electrical conductivities of the two composites evidently increased with increasing temperature. As the temperature increased from 25 °C to 600 °C, the conductivity of SiCf/SiC changed from 0.26 S/m to 0.52 S/m, whereas that of the SiCf/SiC-SiCnw composites was markedly enhanced from 0.88 S/m to 1.77 S/m. In addition, the conductivity of the SiCf/SiC-SiCnw composites was higher than that of the SiCf/SiC composites at each temperature spot. In this work, the electrical conductivity of the SiC matrix was 0.09 S/m measured by twoprobe method. It was difficult to measure the conductivity of the SiC nanowire, hence, we put the nanowires (50 mg) into a polytetrafluoroethylene cylinder with a flat bottom electrode. The measurements were conducted as the sample in the cylinder was compressed at 100, 110 and 120 MPa, respectively, by an upper piston electrode. The measured values were affected by porosity in the sample, so the sample was compressed with such large pressures to eliminate the effect of porosity. The experimental values of the electrical conductivity at different pressures were identical, which were all 12.2 S/m. Therefore, it was concluded that porosity infinitely approached zero by such large compression and the experimental value could represent the electrical conductivity of the sample without the influence of porosity. This meant that the conductivity of SiC nanowires in this work was 12.2 S/m. Obviously, the conductivity of the SiC nanowires was much higher than that of the SiC matrix. To further confirm the positive effect of SiC nanowires on electrical conductivity, SiCf-SiCnw/paraffin composites and SiCf/paraffin composites were prepared, in which paraffin was used as matrix to replace the SiC matrix. The conductivity of the SiCfSiCnw/paraffin composites was 0.69 S/m which was higher than that of the SiCf/paraffin composites (0.13 S/m). These results indicated that the temperature-dependent electrical conductivity was effectively improved by the incorporation of SiC nanowires. This phenomenon can be explained by the unique microstructure of SiC nanowires. Fig. 6(a) shows the schematic illustration of the microstructure and electronic transport modes of the SiC nanowire. The direction of the

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electric field was assumed to be parallel to the SiC nanowire along the axial direction, and the electrons had a dominant drifted direction opposed to the electric field. As shown in Fig. 6(a), the crystal planes were perpendicular to the axis. Thus, electrons jumping across the basal planes were the primary transport mode, thereby indicating that the electrical conductivity was determined by the hopping conductance. This case differed from the transport modes in carbon nanotube (CNT), where both the migrating conductance in the graphene planes and the hopping conductance among the graphene layers exist[39]. Electrons hopping from one plane to another must overcome the potential energy existing among the planes, so the hopping conductance will be lower than the migrating conductance. Unlike the electrons transported in the axial direction, the electrons along the radial direction can easily move. Therefore, the SiC nanowire has a suitable electrical conductivity, which is beneficial for the composites to obtain an appropriate permittivity and impedance match between the composites and the free space. These findings implied that the hopping conductance is the main contributor to the electrical conductivity of the SiC nanowire. In the present case, the conductivity decided mainly by the hopping electrons can be expressed in Eq. (1): 𝜎(𝑇) = 𝐴𝑒 −𝑈⁄𝑘𝑇 (1) where A is a constant, U is the potential barrier, and k is the Boltzmann constant. As indicated in Eq. (1), the conductivity caused by the hopping electrons was enhanced by increasing the temperature. Further, the concentration of electrons in the thermally activated SiC nanowire increased and the electrons were excited to a higher energy state by obtaining the thermal energy with increasing temperature. More electrons in the high-energy state can jump over the potential barriers for transport in the nanowires, leading to higher electrical conductivity with increasing temperature. Aside from the microstructural effects, other factors affecting the electrical conductivity were also considered. The SiC nanowires in the composites are schematically illustrated in Fig. 6(b). Regarding their high aspect ratio, highly intertwined SiC nanowires formed conductive networks and provided additional flow paths in the electrical current within the matrix. Though the SiC nanowires were coated with an insulating layer, a significant tunneling current could flow through the SiO2 layer when the oxide thickness was ultra small [40,41]. In our work, the SiO2 layer coated on the SiC nanowires had a thickness of 3~5 nm which was small enough for tunneling processes. At such small thickness, a substantial direct tunneling current flowed though the layer and the electrons could tunnel in the oxide layer. The contact conductivity decided mainly by the electrons hopping between the interface of the SiC nanowires significantly contributed to the electrical conductivity, which can be enhanced by

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increasing the temperature [42]. Therefore, the electrical conductivity of the SiCf/SiC-SiCnw matrix was higher than that of the SiCf/SiC matrix because of the introduction of SiC nanowires into SiC matrix. Moreover, pores in the matrix contained nonconductive gas and increased tortuosity of the current path in the matrix. The decrease of porosity inside the material could increase the electrical conductivity of the SiCf/SiC-SiCnw composites. Consequently, the electrical conductivity of the SiCf/SiCSiCnw composites was higher than that of the SiCf/SiC composites and enhanced with the elevated temperature, significantly affecting the dielectric loss of the composites at high temperatures.

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3.5 Dielectric properties of SiCf/SiC composites at room temperature Fig. 7 shows the real (ε′) and imaginary (ε′′) parts of the complex permittivity and tanδ of the SiCf/SiC and SiCf/SiC-SiCnw composites as the frequency function of the X-band at room temperature. Both SiCf/SiCSiCnw and SiCf/SiC displayed a typical frequency-dependent permittivity. ε′ and ε′′ of the samples decreased with increasing frequency, which was interpreted as the hysteresis between the displacement current and the build-up potential when the frequency increased [39]. As shown in Figs. 7(a) and 7(b), ε′ and ε′′ of SiCf/SiC-SiCnw are higher than those of SiCf/SiC. The average value of ε′ increased from 1.83 to 4.82, whereas the average ε′′ value changed from 0.54 to 1.71, indicating that the permittivity of composites was markedly enhanced by the incorporation of SiC nanowires. Specifically, small amplitude fluctuations were further observed in both the ε′ and ε′′ curves of the composites over the frequency range. Such behaviour indicated that the typical characteristic of the nonlinear resonant behaviour was due to the dielectric relaxation arising from dipole polarisations in the composites [43]. Fig. 7(c) shows the tanδ of the composites versus the frequency, which represents the loss amount and is calculated using tanδ = ε′′/ε′. Generally, a high tanδ indicates higher loss and better microwave attenuation capability. As shown in Fig. 7(c), the tanδ curves of the two composites present a nonlinear change, which differed from the change trend of the permittivity. The tanδ values of SiCf/SiC-SiCnw were considerably greater than those of SiCf/SiC, owing to the conductance and interface polarisation caused by the increased electrical conductivity and interfaces between the SiC nanowires and the matrix. Hence, SiCf/SiC-SiCnw exhibited a good potential for microwave absorption. The complex permittivity of a material can be described by the following equation [44]: 𝜀 ′ = 𝜀 ′ − 𝑗𝜀 ′′

(2)

According to Debye theory, the real and imaginary permittivities can be

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calculated using the following equations: 𝜀𝑠 − 𝜀∞ 𝜀 ′ = 𝜀∞ + (3) 1 + 𝜔2𝜏2 (𝜀𝑠 − 𝜀∞ )𝜔𝜏 𝜎 𝜀 ′′ = + = 𝜀 ′′ 𝑟𝑒𝑙𝑎𝑥 + 𝜀 ′′ 𝜎 (4) 2 2 1+𝜔 𝜏 𝜔𝜀0 where εs is the static permittivity, ε∞ is the relative dielectric permittivity at the high-frequency limit, ε0 is the vacuum dielectric constant, ω is the angular frequency, τ is the temperature-dependent relaxation time, σ is the temperature-dependent electrical conductivity, ε′′relax is the relaxation polarisation loss and ε′′σ is the electrical conductivity loss. ε′ represents the polarisation capacity of a material. For SiCf/SiC-SiCnw composites, the introduction of SiC nanowires generated abundant grain boundaries, high density of stacking faults within the SiC nanowires, and extensive interfaces between the SiC nanowires and SiC matrix. The charges accumulated in the interfaces can form space charge layers, leading to an interfacial polarisation under the external electric fields. Thus, ε′ of SiCf/SiC-SiCnw was higher than that of SiCf/SiC. ε′′ is the capacity of dielectric loss. The dipole polarisation relaxations induced by the electron motion hysteresis in the dipole under alternating EM field enhanced the dielectric loss. Apart from the polarization, the conductive loss was another factor that contributed to the dielectric loss. The SiC nanowires had a high carrier concentration, and confining carriers into one-dimensional nanowires on a nanometer scale can significantly increase the carrier concentration, which is conducive for high conductivity. Meanwhile, the conductive network formed by SiC nanowires provided channels for mobile charge carriers and interacted with the EM field[31]. Therefore, both the relaxation polarisation loss and the electrical conductivity loss were enhanced by the induction of SiC nanowires, consequently increasing the ε′′ values of the composites. According to Eqs. (3) and (4), the relationship between ε′ and ε′′ can be deduced as

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𝜀𝑠 + 𝜀∞ 2 𝜀𝑠 − 𝜀∞ 2 ′′ 2 (5) (𝜀 − ) + (𝜀 ) = ( ) 2 2 The plot of ε′′ versus ε′ is a single semicircle, called the Cole–Cole plot. This semicircle corresponds to one Debye relaxation process [45]. Fig. 8 shows the ε′′–ε′ curves of the two composites in the X-band. The two composite curves contained semicircles and a tail, indicating the existence of multiple dielectric relaxation processes under an alternating EM field and the influence of conductance. The interfaces introduced by the SiC nanowires in the SiCf/SiC-SiCnw composites contributed to the relaxation and polarization process. Thus, the semicircles in the Cole–Cole curves of the SiCf/SiC-SiCnw were more evident than those of the SiCf/SiC ′

µr 2𝜋𝑓𝑑 Zin = Z0 √ tan h [𝑗 ( ) √µr 𝜀r ] 𝜀r 𝑐

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composites. To further evaluate the EM absorption capability of the as-received composites, the reflection loss (RL) was calculated from the relative permittivity and permeability at a given frequency and the thickness layer according to the transmission line theory. The RL can be expressed in the following equations [46]: Zin − Z0 RL = 20log | (6) | Zin + Z0

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where Zin is the input impedance of the absorber; Z0 is the impedance of air; εr and μr are the relative complex permittivity and permeability of the absorber, respectively; ƒ is the frequency of the microwaves; d is the thickness of the absorber; and c is the velocity of microwaves in free space. In our calculation, d was considered to be 2–5 mm. Here, μr was assumed to be 1, given the non-magnetic properties of the SiCf/SiC-SiCnw composites. The RL is an effective parameter for evaluating the microwave absorption performance of materials. A lower RL value indicates a better microwave absorption performance. When the RL value is lower than −10 dB, less than 10% of the microwave energy is reflected, whereas more than 90% of the microwave power is absorbed. The corresponding frequency range within which RL ≤ −10 dB is defined as the effective absorption bandwidth (EAB). The variations in the RL values of the SiCf/SiC-SiCnw and SiCf/SiC composites for different thicknesses as a function of the frequency of the X-band are shown in Figs. 9(a) and 9(b), respectively. The SiCf/SiC composites exhibited a poor microwave absorption performance at a random frequency and thickness. No evident peak values were observed, and the average RL values decreased from −0.3 dB to −4.3 dB as the thickness increased from 2 mm to 5 mm. No effective absorption was detected in the entire thickness of the SiCf/SiC composites at the X-band owing to its low dielectric loss. Compared with that of the SiCf/SiC composites, the RL values of the SiCf/SiC-SiCnw composites with the same thickness significantly increased because of the introduction of SiC nanowires, thereby exhibiting a high-performance EM wave absorption capability. The minimum RL (RLmin) of the SiCf/SiC-SiCnw composites was −16.5 dB at 8.2 GHz when the absorber thickness was 4.5 mm. Moreover, the EAB was 1.3 GHz (8.2–9.5 GHz). These results indicated that the SiCf/SiC-SiCnw composites exhibited not only high RL values but also a wide EAB. In addition, the RLmin values of the composites changed with

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increasing thickness, thereby indicating that the range of the absorption frequency can be tuned by adjusting the thickness of the absorbers. The RLmin values shifted to a lower frequency when the thickness increased because of the resonant absorption caused by the quarter-wavelength attenuation. Fig. 10 shows a schematic model of the single-layer absorber on the metal back panel, illustrating the peak shift with the variation in thickness. As shown in Fig. 10(a), a certain microwave with a wavelength of λ1 propagated into the absorbing layer with a thickness of d1, and an angular phase difference existed between the microwave reflected from air– absorber interfaces and the microwave reflected from the absorber–metal back plane. When the absorber thickness (d1) was an odd multiple of a quarter of the wavelength, the angular phase difference was 180°, thereby causing a destructive interference. Here, a strong reflection loss occurred, which corresponded to RLmin. When the thickness increased from d1 to d2, the phase matching condition was destroyed, as shown in Fig. 10(b). This phenomenon indicated that a destructive interference cannot occur owing to the phase mismatch of the upper and bottom surface reflected waves, leading to a weak loss. To satisfy the phase match condition again, the waves (λ1) were adjusted to waves (λ2) by increasing the wavelength when the thickness was d2, as shown in Fig. 10(c). Under this condition, a strong reflection loss occurred again because the upper surface once again had a phase opposite to that from the bottom surface. The peak frequency corresponding to RLmin can be described as Eq. (8):

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nc (8) 4d√µr εr where fm is the peak frequency of RLmin. Therefore, as depicted by the schematic model in Figs. 10(a)–10(c), an increasing thickness caused the RL peaks to shift to a lower frequency, according to Eq. (8).

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3.6 Dielectric properties of the SiCf/SiC composites at high temperatures To evaluate the high-temperature microwave absorption performance of the SiCf/SiC-SiCnw composites, the relative complex permittivity and tanδ at high temperatures were examined. Fig. 11 exhibits ε′, ε′′ and tanδ of the SiCf/SiC-SiCnw and SiCf/SiC composites at different temperatures ranging from 25 °C to 600 °C in the frequency range of 8.2–12.4 GHz. As shown in Figs. 11(a)–11(f), both ε′ and ε′′ of the two composites apparently decrease with increasing frequency and increase with rising temperature. The tanδ of the composites tended to increase with increasing temperature, similar to that of the complex permittivity. For the SiCf/SiC composites, the average values of ε′ and ε′′ changed from 1.83 to 3.03 and from 0.54 to 1.14, respectively, and tanδ increased from 0.29 to 0.37 when the

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temperature increased from 25 °C to 600 °C. Unlike the SiCf/SiC composites, the SiCf/SiC-SiCnw composites exhibited relatively high values of ε′, ε′′ and tanδ, which had average values of 4.82, 1.71 and 0.35, respectively, at 25 °C. As the temperature reached 600 °C, a significant increase in ε′ and ε′′ (nearly 61% and 109%, respectively) was observed. The SiCf/SiC-SiCnw composites showed higher enhanced tendencies and amplitudes in both complex permittivity and tanδ at each temperature spot than the SiCf/SiC composites. Therefore, the dielectric properties of the SiCf/SiC composites can be significantly enhanced by the introduction of SiC nanowires, especially at high temperatures. To particularly demonstrate the temperature dependence of the complex permittivity and the tangent loss, Fig. 12 shows the dielectric properties versus temperature plots at the five selected frequencies (i.e., 8.2, 9.2, 10.2, 11.2 and 12.2 GHz). The ε′, ε′′ and tanδ values showed strong temperature-dependent effects in all of the measured frequencies and displayed an increasing trend. Thus, the dielectric properties of the composites significantly exhibited temperature dependence at 25–600 °C. Debye theory can be applied to analyse the temperature variation of the dielectric properties of the composites. According to Eqs. (5) and (6), ε′ and ε′′ were determined by the relaxation time and the electrical conductivity, which varied with temperature. As mentioned in previous studies [47,48], the polarisation in SiC materials was mainly caused by the electron relaxation polarisation in response to the EM field at high temperatures caused by the thermal excitation of SiC materials. The relationship between the temperature and polarisation relaxation time of the composites can be described by the Arrhenius formula as follows [49]: U (9) ) RT where τ0 is the prefactor, T is the temperature, and U is the activation energy. By substituting Eq. (9) into Eq. (3), ε′ can be rewritten as a function of temperature: 𝜀𝑠 − 𝜀∞ 𝜀 ′ = 𝜀∞ + (10) 2𝑈⁄ 2 2 𝑅𝑇 1 + 𝜔 𝜏0 𝑒

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Based on Eqs. (9) and (10), the relaxation time of the electron relaxation polarisation decreased with increasing temperature, thereby increasing ε′. The electrons in the composites were believed to respond faster to the alternating EM field when the temperature increased, which caused a shorter relaxation time during polarisation [50]. According to Eq. (4), both the relaxation polarisation loss and the electrical conductivity loss contributed to ε′′ of the composites. To investigate the individual contribution of the two loss mechanisms to ε′′,

we compared the experimental data of ε′′ and the conductivity loss of ε′′σ. ε′′σ can be calculated using the following equation based on the temperature-dependent electrical conductivity: 𝜀 ′′ 𝜎 =

𝜎(𝑇) 𝜎(𝑇) = 𝜔𝜀0 2𝜋𝑓𝜀0

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Fig. 13 shows the comparison of the measured ε′′ and calculated ε′′σ of the two composites at given frequencies of 9 and 11 GHz at various temperatures. The calculated ε′′σ and the measured ε′′ displayed a similar increasing tendency with increasing temperature at the two given frequencies for both composites. The calculated ε′′σ was comparable to the measured ε′′ in the experimental temperature range. The conductivity loss was revealed to be dominant in the dielectric loss, although the relaxation loss caused the departure between ε′′σ and ε′′. Therefore, the increasing ε′′ of the composites was attributed to the increase in the electrical conductivity with rising temperature. To further investigate the influence of temperature on the microwave absorption of the SiCf/SiC composites, the high-temperature RL was also calculated. Fig. 14 shows the maps of the evaluated RL values of the composites coupled with varying thickness. The minimum absorption peak of the SiCf/SiC composites was −11.8 dB at 9.7 GHz in 600 °C with a thickness of 5 mm, whereas that of the SiCf/SiC-SiCnw composites exhibited a more effective microwave absorption with a RLmin of −47.5 dB at 11.4 GHz and a thickness of 2.5 mm. This result demonstrated that the absorption ability of the SiCf/SiC composites can be enhanced by the incorporation of SiC nanowires. The RL of the two composites increased with increasing temperature. These findings indicated that the microwave absorption ability of the composites at different frequencies can be effectively tuned by adjusting the thickness of the absorbers and temperature. To analyse the microwave absorption performance in-depth, the RL values for the thickness of 3 mm was used as an example. Fig. 15 shows the RL plots versus the frequency and temperature of the two composites. As shown in Figs. 15(a) and 15(b), the SiCf/SiC composites had a poor absorption property with a RLmin of −4.9 dB, which was much lower than that of the SiCf/SiC-SiCnw composites with maximum absorption peaks of up to −28.3 dB. Unlike the SiCf/SiC composites, the SiCf/SiC-SiCnw composites were more effective in microwave absorption. This advantage was mainly attributed to the enhanced complex permittivity of the SiCf/SiC-SiCnw composites, leading to a better microwave absorption property. In addition, the maximum absorption peaks of the SiCf/SiC-SiCnw composites was approximately 11.2 GHz at 200 °C and 9.3 GHz at 600 °C, indicating that the maximum absorption peak slightly shifted to a lower

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frequency with increasing temperature. This phenomenon was also attributed to the law of quarter-wavelength attenuation. When the complex permittivity increased with increasing temperature, the peak shifted to a lower frequency according to Eq. 8. In Figs. 15(c) and 15(d), the RL– temperature curves at five different frequencies demonstrated that the composites showed strong temperature-dependent effects on the microwave absorption property, which can be explained by the change in the complex permittivity and the impedance matching condition between the composites and the free space as the temperature increased. The detailed minimum RL values, peak frequency and bandwidth of the two composites are presented in Table 2. Unlike the SiCf/SiC composites, the SiCf/SiC-SiCnw composites showed a broader EAB of 0.3 GHz, and the RLmin value was −10.2 dB at approximately 12.4 GHz with 25 °C. The RLmin value reached up to −28.3 dB at 9.3 GHz when the temperature increased to 600 °C, and the corresponding bandwidth below −10 dB extended to 2.9 GHz (from 8.2 GHz to 11.1 GHz). The EAB for the composites showed an increasing trend with increasing temperature. The results indicated that the composites exhibited not only high RL values but also a wide absorption bandwidth. Therefore, temperature is critical in enhancing the microwave absorption properties. Moreover, the outstanding absorption properties of the SiCf/SiC-SiCnw composites suggest that this type of composite can be used as promising structural and functional materials for microwave absorption applications at high temperatures.

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3.7 Microwave absorption mechanisms of the SiCf/SiC-SiCnw composites The schematic illustration in Fig. 16 shows the potential absorption mechanisms of the SiCf/SiC-SiCnw composites with high-performance microwave absorption. First, when the incident microwaves interacted with the composites, a few waves were reflected to free space, whereas other waves infiltrated into the composites because of the appropriate impedance matching condition. The three-dimensional hierarchical system caused the incident microwave to generate multiple reflections between the different SiC nanowires and fibres, thereby increasing the microwave propagation path and the possibility of attenuation of the microwave. The incident waves restricted in the composites were repeatedly scattered and reflected until they were consumed and absorbed, consequently enhancing the microwave energy consumption capacity of the composites. Second, onedimensional SiC nanowires with a large aspect ratio can easily create a conductive network in the composites. Hopping electrons can jump across the interface among nanowires, and energy will be induced into the microcurrent in the network if an external electric field is applied to the material, possibly generating a strong conductive loss and enhancing the EM absorption capability [51,52]. In addition, the dipole polarisation

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originating from the SiCnw/SiC and SiCnw/SiCnw interfaces and the nanograin boundaries, which have numerous defects, such as vacancies and dislocations, is another important factor in EM dissipation. The accumulation of free charges at various interfaces and abundant boundaries will change the charge density and cause an electric dipole moment when the composites are exposed to an alternating electronic field, which will generate an interfacial polarisation relaxation. Importantly, multiple polarisations arising from the high-density stacking faults in the SiC nanowires play a significant role in the enhanced EM dissipation. The polarisations cannot match with the change in the EM field at high frequency and require a long relaxation time, which converts much energy into heat, thereby enhancing the dielectric loss and the EM energy dissipation. These results suggested that the incorporation of SiC nanowires can improve the microwave absorption performance of the SiCf/SiC composites. Here we summarised the RL values of the prepared composites in this work and other representative works in open literature, as shown in Table 3. The results in this work are comparable with those representative materials in microwave absorption properties, especially when used at high temperatures. Most importantly, the prepared composites with continuous reinforced fibers have considerable advantages for the industrial applications as a structural functional materials owing to their excellent mechanical and oxidation resistance properties. Thus, compared with other ceramics and composites, the SiCf/SiC-SiCnw composites can be the most potential candidate by simultaneously satisfying the requirements for high mechanical and excellent microwave absorption properties, which can be used in harsh environments.

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4. Conclusion In this study, the SiCf/SiC-SiCnw composites were successfully synthesised by the introduction of SiC nanowires on the SiC fibre through an in situ grown method and subsequent PIP of the SiC matrix onto threedimensional preforms. The results indicated that the SiCf/SiC-SiCnw composites exhibited a temperature-dependent complex permittivity and tanδ, as the temperature increased from 25 °C to 600 °C. Compared with that of the SiCf/SiC composites, the RLmin value of the SiCf/SiC-SiCnw composites reached up to −47.5 dB at 11.4 GHz with a broad EAB of 2.8 GHz for the thickness of 2.5 mm when the temperature was 600 °C. This result demonstrated that SiC nanowires can greatly enhance the microwave absorption and improve the broadband absorption, particularly at high temperatures. The excellent microwave absorption performance of the composites was attributed to the synergistic effects of multiple reflections, various polarisations, destructive interference, and conductive network.

These findings suggested that SiCf/SiC composites with SiC nanowires can be used as promising high-performance microwave absorption materials in high-temperature environments.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21071011).

References

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[1] B. Wen, M. Cao, M. Lu, W. Cao, H. Shi, J. Liu, X. Wang, H. Jin, X. Fang, W. Wang, Reduced graphene oxides: light-weight and highefficiency electromagnetic interference shielding atelevated temperatures, Adv. Mater. 26 (21) (2014) 3484–3489. [2] Z. Chen, C. Xu, C. Ma, W. Ren, H. M. Cheng, Light weight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (2013) 1296–1300. [3] Q. Li, X. Yin, W. Duan, L. Kong, X. Liu, L. Cheng, L. Zhang, Improved dielectric and Electromagnetic interference shielding properties of ferrocene-modified polycarbosilane derived SiC/C composite ceramics, J. Eur. Ceram. Soc. 34 (2014) 2187–2201. [4] S.S.S. Afghahi, M. Jafarian, C.A. Stergiou, Multicomponent nanocomposites with carbonyl Fe-CoFe2O4-CaTiO3 fillers for microwave absorption applications, Mater. Design 112 (2016) 462–468. [5] D. Micheli, R.B. Morles, M. Marchetti, F. Moglie, V.M. Primiani, Broadband electromagnetic characterization of carbon foam to metal contact, Carbon 68 (2014) 149–158. [6] M.S. Cao, X.X. Wang, W.Q. Cao, J. Yuan, Ultrathin Graphene: Electrical properties and highly efficient electromagnetic interference shielding, J. Mater. Chem .C 3 (2015) 6589−6599. [7] Y. Jia, K. Li, L. Xue, J. Ren, S. Zhang, H. Li. Mechanical and electromagnetic shielding performance of carbon fiber reinforced multilayered (PyC-SiC)n matrix composites, Carbon 111(2017) 299–308. [8] D.H. Kim, Y. Kim, J.W. Kim, Transparent and flexible film for shielding electromagnetic interference, Mater. Des. 89 (2016) 703–707. [9] X. Yin, L. Kong, L. Zhang, L. Cheng, N. Travitzky, P. Greil, Electromagnetic properties of Si-CN based ceramics and composites, Int. Mater. Rev. 59 (2014) 326–355. [10] Y. Zhang, Y. Huang, T. Zhang, H. Chang, P. Xiao, H. Chen, Z. Huang, Y. Chen, Broad band and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam, Adv. Mater. 27 (12) (2015) 2049–2053. [11] X. Liu, Z. Zhang, Y. Wu, Absorption properties of carbon black/silicon carbide microwave absorbers, Compos. Part B Eng. 42 (2011) 326–329. [12] Y. Wang, W. Zhang, C.Luo, X. Wu, Q. Wang, W. Chen, J. Li, Synthesis, characterization and enhanced electromagnetic properties of NiFe2O4@SiO2-decorated reduced graphene oxide nanosheets, Ceram. Int. 42 (2016) 17374–17381. [13] I.M. DeRosa, A. Dinescu, F. Sarasini, M.S. Sarto, A.Tamburrano, Effect of short carbon fibers and MWCNTs on microwave absorbing

A

CC E

PT

ED

M

A

N

U

SC R

IP T

properties of polyester composites containing nickel-coated carbon fibers, Compos. Sci. Technol. 70 (1) (2010) 102–109. [14] Y. Liu, Y.N. Li, K.D. Jiang, G.X. Tong, T.X. Lv, W.H. Wu, Controllable synthesis of elliptical Fe3O4@C and Fe3O4/Fe@C nanorings for Plasmon resonance-enhanced microwave absorption, J. Mater. Chem. C 4 (2016) 7316–7323. [15] S. Qiu, H.L. Lyu, J.R. Liu, Y.Z. Liu, N.N. Wu, W. Liu, Facile synthesis of porous Nickel/Carbon composite microspheres with enhanced electromagnetic wave absorption by magnetic and dielectric losses. ACS Appl. Mater. Interfaces 8 (2016) 20258−20266. [16] H. Xu, X.W. Yin, M. Zhu, M.K. Han,; Z.X. Hou, X.L. Li, L.T. Zhang, L.F. Cheng, Carbon hollow microspheres with a designable mesoporous shell for high performance electromagnetic wave absorption. ACS Appl. Mater. Interfaces 9 (2017) 6332−6341. [17] L. B. Kong, Z. W. Li, L. Liu, R. Huang, M. Abshinova, Z. H. Yang, C. B. Tang, P. K. Tan, C. R. Deng, S. Matitsine, Recent progress in some composite materials and structures for specific electromagnetic applications, Int. Mater. Rev. 58 (2013) 203–259. [18] X. W. Yin, L. F. Cheng, L. T. Zhang, N. Travitzky, P. Greil, Fiberreinforced multifunctional SiC matrix composite materials, Int. Mater. Rev. 62 (2017) 117–172. [19] Q. Li, X. Yin, W. Duan, L. Cheng, L. Zhang, Improved dielectric properties of PDCs-SiCN by in-situ fabricated nano-structured carbons, J. Eur. Ceram. Soc. 37 (2017) 1243–1251. [20] A. Guo, M. Roso, M. Modesti, J. Liu, P. Colombo, Hierarchically structured polymer-derived ceramic fibers by electrospinning and catalystassisted pyrolysis, J. Eur. Ceram. Soc. 34 (2014) 549–554. [21] F. Ye, L. Zhang, X. Yin, Y. Zhang, L. Kong, Y. Liu, L. Cheng, Dielectric and microwave-absorption properties of SiC nanoparticle/SiBCN composite ceramics, J. Eur. Ceram. Soc. 34 (2014) 205–215. [22] A. Ivekovic, S. Novak, G. Dražic, D. Blagoeva, S. Gonzalez de Vicente, Current status and prospects of SiCf/SiC for fusion structural applications, J. Eur. Ceram. Soc. 33 (2013) 1577–1589. [23] Linus U.J.T. Ogbuji, Pest-resistance in SiC/BN/SiC composites, J. Eur. Ceram. Soc. 23 (2003) 613–617. [24] Y. Mu, W.C. Zhou, Y. Hu, H.Y. Wang, F. Luo, D.H. Ding, Y.C. Qing, Temperature-dependent dielectric and microwave absorption properties of SiCf/SiC-Al2O3 composites modified by thermal cross-linking procedure, J. Eur. Ceram. Soc. 35 (2015) 2991−3003. [25] Z.H. Hou, R.Y. Luo, W. Yang, H.Z. Xu, T. Han, Effect of interface type on the static and dynamic mechanical properties of 3D braided SiCf/SiC composites, Mater. Sci. Eng. A 669 (2016) 66–74.

A

CC E

PT

ED

M

A

N

U

SC R

IP T

[26] Z.H. Hou, R.Y. Luo, W. Yang, H.Z. Xu, T. Han, Effect of fiber directionality on the static and dynamic mechanical properties of 3D SiCf/SiC composites, Mater. Sci. Eng. A 658 (2016) 263–271. [27] R.B. Wu, Z.H. Yang, M.S. Fu, K. Zhou, In-situ growth of SiC nanowire arrays on carbon fibers and their microwave absorption properties, J. Alloys Compd. 687 (2016) 833−838. [28] W. Duan, X. Yin, Q. Li, X. Liu, L. Cheng, L. Zhang, Synthesis and microwave absorption properties of SiC nanowires reinforced SiOC ceramic, J. Eur. Ceram. Soc. 34 (2014) 257–266. [29] J. Men, Y. Liu, R. Luo, W. Li, L. Cheng, L. Zhang, Growth of SiC nanowires by low pressure chemical vapor infiltration using different catalysts, J. Eur. Ceram. Soc. 36 (2016) 3615–3625. [30] L.W. Yan, C.Q. Hong, B. Q. Sun, G.D. Zhao, Y.H. Cheng, S. Dong, D.Y. Zhang, X.H. Zhang, In situ growth of core−sheath heterostructural SiC nanowire arrays on carbon fibers and enhanced electromagnetic wave absorption performance, ACS Appl. Mater. Interfaces 9 (2017) 6320–6331. [31] J.L. Kuang, W.B. Cao, Stacking faults induced high dielectric permittivity of SiC Wires. Appl. Phys. Lett. 103 (2013) 112906. [32]Y.Y. Choi, S.J. Park, D.J. Choi, Gas-phase synthesis and growth mechanism of SiC/SiO2 core-shell nanowires, CrystEngComm, 14 (2012) 1737–1743. [33] Y.S. Liu, J. Men,W. Feng, L.F. Cheng, L.T. Zhang, Catalyst-free growth of SiC nanowires in a porous graphite substrate by low pressure chemical vapor infiltration, Ceram. Int. 140 (2014)11889–11897. [34] J.X. Dai, J.J. Sha, Z.F. Zhang, Y.C. Wang, W. Krenkel, Synthesis of high crystalline beta SiC nanowires on a large scale without catalyst, Ceram. Int. 41(2015) 9637–9641. [35] A.C. Ferrari, J. Robertson, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2009) 187401–187404. [36] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2000) 14095–14107. [37] M. Bechelany, A. Brioude, D. Cornu, G. Ferro, P. Miele, A Raman spectroscopy study of individual SiC nanowires, Adv. Funct. Mater.17 (6) (2007) 939–943. [38] S. Rohmfeld, M. Hundhausen, L. Ley, Raman scattering in polycrystalline 3C-SiC: Influence of stacking faults, Phys. Rev. B 58 (1998) 9858–9862. [39] M.S. Cao, W. Song, Z.L. Hou, B. Wen, J. Yuan, The effects of temperature and frequency on the dielectric properties, electromagnetic interference shielding and microwave-absorption of short carbon fiber/silica composites, Carbon 48 (2010)788–796. [40] A. Ghetti, Characterization and modeling of the tunneling current in Si-SiO2-Si structures with ultra-thin oxide layer, Microelectron. Eng. 59

A

CC E

PT

ED

M

A

N

U

SC R

IP T

(2001) 127–136. [41] M. Roczen, E. Mglguth, M. Schade, A. Schopke, A. Laades, Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications, J. Non-Cryst. Solids, 358 (2012) 2253–2256. [42] B. Wen, M.S. Cao, Z.L. Hou, W.L. Song, L. Zhang, M.M. Lu, H.B. Jin, X.Y. Fang, W.Z. Wang, J. Yuan, Temperature dependent microwave attenuation behavior for carbon-nanotube/silica composites, Carbon 65 (2013) 124–139. [43] P.C. Watts, W.K. Hsu, A. Barnes, B. Chambers, High permittivity from defective multiwalled carbon nanotubes in the X-band, Adv. Mater. 15 (2003) 600–603. [44] B. Zhong, T.Q. Sai, L. Xia, Y.L. Yu, G.W. Wen, High-efficient production of SiC/SiO2 core-shell nanowires for effective microwave absorption, Mater. Des. 121 (2017) 185–193. [45] H.J. Yang , J. Yuan ,Y. Li, Z.L. Hou, H.B. Jin, X.Y. Fan, M.S. Cao, Silicon carbide powders: Temperature-dependent dielectric properties and enhanced microwave absorption at gigahertz range, Solid State Commun. 163 (2013) 1–6. [46] M.K. Han, X.W. Yin, W.Y. Duan, S. Ren, L.T. Zhang, L.F. Cheng, Hierarchical graphene/SiC nanowire networks in polymer-derived ceramics with enhanced electromagnetic wave absorbing capability, J. Eur. Ceram. Soc. 36 (2016) 2695–2703. [47] E. Volz, A. Roosen, W. Hartung, A. Winnacker, Electrical and thermal conductivity of liquid phase sintered SiC, J. Eur. Ceram. Soc. 21 (2001) 2089–2093. [48] T.S. Orlova, V.V. Popov, J. Quispe Cancapa, D. Hernández Maldonadob, E. Enrique Magarinob, F.M. Varela Feriab, A. Ramírez de Arellanob, J. Martínez Fernándezb, Electrical properties of biomorphic SiC ceramics and SiC/Si composites fabricated from medium density fiberboard, J. Eur. Ceram. Soc. 31(2011) 1317–1323. [49] N.T. Correia, J.J. MouraRamos, On the cooperativity of the betarelaxation: A discussion based on dielectric relaxation and thermally stimulated depolarization currents data, Phys. Chem. Chem. Phys. 2 (2000) 5712–5715. [50] W.L. Song, M.S. Cao, Z.L. Hou, J. Yuan, X.Y. Fang, High-temperature microwave absorption and evolutionary behavior of multiwalled carbon nanotube nanocomposite, Scripta Mater. 61 (2009) 201–204. [51] P. Wang, L. F. Cheng, L. T. Zhang, One-dimensional carbon/SiC nanocomposites with tunable dielectric and broadband electromagnetic wave absorption properties, Carbon 125 (2017) 207–220. [52] P. Wang, L. F. Cheng, Y. N. Zhang, L. T. Zhang, Flexible SiC/Si3N4 composite nanofibers with in situ embedded graphite for highly efficient

A

CC E

PT

ED

M

A

N

U

SC R

IP T

electromagnetic wave absorption, ACS Appl. Mater. Interf. 9 (2017) 28844–28858. [53] Y.J. Zhang, X.W. Yin, F. Ye, L. Kong, Effects of multi-walled carbon nanotubes on the crystallization behavior of PDCs-SiBCN and their improved dielectric and EM absorbing properties, J. Eur. Ceram. Soc. 34 (2014) 1053–1061. [54] W.Y. Duan, X.W. Yin, F.X. Cao, Y.L. Jia, Y. Xie, P. Greil, N. Travitzky, Absorption properties of twinned SiC nanowires reinforced Si3N4 composites fabricated by 3D-prining, Mater. Lett. 159 (2015) 257–260. [55] D. Micheli, C. Apollo, R. Pastore, M. Marchetti, X-Band microwave characterization of carbon-based nanocomposite material, absorption capability comparison and RAS design simulation, Compos. Sci. Technol. 70 (2010) 400–409. [56] P. Wang, L. F. Cheng, Y. N. Zhang, W. Y. Yuan, H. X. Pan, H. Wu, Electrospinning of graphite/SiC hybrid nanowires with tunable dielectric and microwave absorption characteristics, Compos. Part A 104 (2018) 68– 80. [57] H. Zhang, Y. Xu, J. Zhou, J. Jiao, Y. Chen, H. Wang, Stacking faults and unoccupied densities of state dependence of electromagneticwave absorption in SiC nanowires, J. Mater. Chem. C 3 (2015) 4416–4423.

Figure captions

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Fig. 1. High-temperature complex permittivity test apparatus: (1) computer, (2) vector network analyzer, (3) temperature controller, (4) coaxial cable, (5) waveguide test chamber, (6) sample, (7) heater, and (8) cooling system.

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Fig. 2. (a) SEM image of the pristine SiC fibre; (b) low- and (c) highmagnification SEM images of SiC nanowires grown on SiC fibres (right top inset: local magnified image of a nanowire); (d) the corresponding EDS spectrum of the nanowire.

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Fig. 3. (a)TEM image of the SiC nanowire; (b) high-magnification TEM image of the SiC nanowire; (c) HRTEM image of the white square area in (b) and the corresponding SEAD (the right top inset);

Fig. 4. (a) XRD spectra and (b) Raman spectra of the SiC fabric with insitu grown SiC nanowires and the SiC fibre fabric.

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Fig. 6. (a) Schematic illustration exhibiting the microstructure and electronic transport modes of SiC nanowire; (b) a conductive network formed by SiC nanowires in the SiCf/SiC composites.

Fig. 7. (a) Real permittivity, (b) imaginary permittivity, and (c) tangent loss of the SiCf/SiC-SiCnw and SiCf/SiC composites.

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A

CC E

PT

ED

M

A

N

U

SC R

IP T

Fig. 10. Schematic illustration of the reflection loss shift for varying thicknesses.

Fig. 11. (a) Real permittivity, (b) imaginary permittivity, and (c) tangent loss of the SiCf/SiC composites; and (d) real permittivity, (e) imaginary permittivity, and (f) tangent loss of the SiCf/SiC-SiCnw composites at different temperatures.

2.0

8

9

10

11

1.0

0.8

0.6

12

8

9

5

N

4.0

25ºC 200ºC 400ºC 600ºC

A

3.5 3.0 2.5 2.0

0.35

(f)

25ºC 200ºC 400ºC 600ºC

0.30

0.25

12

M

6

8

9

10

11

12

Frequency (GHz)

25ºC 200ºC 400ºC 600ºC

0.50

0.45

0.40

0.35

1.5

8

9

10

11

12

CC E

PT

Frequency (GHz)

A

Real permittivity ()

7

(e) 4.5

ED

25ºC 200ºC 400ºC 600ºC

8

11

Frequency (GHz)

Imaginary permittivtiy ()

9

10

U

Frequency (GHz)

(d)

0.40

IP T

2.4

1.2

(c)

Loss tangent (tan)

2.8

25 ºC 200ºC 400ºC 600ºC

SC R

Real permittivity ()

3.2

(b) 1.4

Loss tangent (tan)

25ºC 200ºC 400ºC 600ºC

Imaginary permittivity ()

(a) 3.6

8

9

10

11

Frequency (GHz)

12

0.30

8

9

10

11

Frequency (GHz)

12

(b)

1.6

0

200

400

0.6

0

200

Temperature (ºC)

(e) Imaginary permittivity ()

7

5

0

200

400

600

CC E

PT

Temperaute (ºC)

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

4

3

0

0.35

0.30

0.25

600

0

200

400

600

Temperature (ºC)

(f)

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

0.50

2

ED

6

A

Real permittivty ()

Temperature (ºC)

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

8

400

M

(d)

Loss tangent (tan)

0.8

0.4

600

SC R

2.0

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

0.40

Loss tangent (tan)

2.4

1.0

N

2.8

(c)

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

1.2

U

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

A

3.2

Imaginary permittivity ()

Real permittivity ()

(a)

IP T

Fig. 12. Dielectric properties vs. temperature at various frequencies: (a) the real permittivity, (b) imaginary permittivity, and (c) tangent loss of the SiCf/SiC composites; and (d) the real permittivity, (e) imaginary permittivity, and (f) tangent loss of the SiCf/SiC-SiCnw composites.

0.45

0.40

0.35

200

400

Temperature (ºC)

600

0

200

400

Temperature (ºC)

600

Fig. 13. Comparison of the measured  and the calculated  vs. the temperature of the SiCf/SiC (a) and SiCf/SiC-SiCnw composites (b) at 9 and 11 GHz.

1.0

11GHz

0.8

0.6

3.5 3.0 2.5 2.0

200

400

N

1.5 0.4

0

600

A

CC E

PT

ED

M

A

Temperature (ºC)

9GHz

IP T

9GHz

measured  calculated 

SC R

1.2

(b) 4.0

11GHz

U

measured  calculated 

Imaginary permittivity ()

Imaginary permittivity ()

(a)

200

400

Temperature (ºC)

600

Fig.14. Microwave absorption properties of the SiCf/SiC (a–d) and SiCf/SiC-SiCnw composites (e–h) vs. the frequency and the thickness at different temperatures.

(a) 5.0

0.00

-5dB

(e) 5.0

0.00

-0.93

-2.75

-5dB

4.5

4.5

-5.50

-3.71

3.5

-4.63

3.0

-5.56

T=25ºC

2.5

-11.00

3.5

-13.75

3.0

-16.50

T=25ºC

2.5 2.0

2.0

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Frequency (GHz)

Frequency (GHz)

(b)

5.0

0.00

(f)

5.0

-1.34

-5.36

3.5

-6.70

3.0

-8.04

4.0

T=200ºC

3.5

2.0

4.0

3.0

PT

3.5

-5dB

CC E

Thickness (mm)

4.5

0.00

Frequency (GHz)

(g)

5.0

-6.27 -12.53

-3.33 -5.00

-8.33 -10.00

T=400ºC

3.5

-18.80 -25.07

-10dB -31.33

3.0

-37.60

T=400ºC

2.5

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Frequency (GHz)

Frequency (GHz) 0.00

(h)

5.0

0.00 -7.93

-1.97

-10dB

4.0

4.5

-5.92

-5dB

-7.90 -9.88

3.0

-11.85

T=600ºC

2.5

-15.87

-3.95

Thickness (mm)

Thickness (mm)

-5dB

4.0

2.0

(d) 5.0

A

0.00

4.5

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

3.5

T=200ºC

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

2.0

4.5

-14.73

-22.10

-1.67

-6.67

2.5

-7.37

2.0

Thickness (mm)

ED

5.0

-3.68

-18.42

3.0

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Frequency (GHz)

0.00

-11.05

2.5

M

2.5

(c)

-5dB

-10dB

N

-4.02

Thickness (mm)

-2.68

4.0

U

4.5

-5dB

A

Thickness (mm)

4.5

-8.25

-10dB

IP T

-2.78

4.0

SC R

4.0

Thickness (mm)

Thickness (mm)

-1.85

4.0

-23.80

-5dB 3.5

-39.67

3.0

-10dB -47.60

T=600ºC

2.5 2.0

2.0

-31.73

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Frequency (GHz)

Frequency (GHz)

Fig. 15. SiCf/SiC composites: (a) reflection loss vs. frequency and (c) reflection loss vs. temperature; SiCf/SiC-SiCnw composites: (b) reflection loss vs. frequency and (d) reflection loss vs. temperature.

0

0

(b)

-3

25ºC 200ºC 400ºC 600ºC

-4 -5 8

-10 -15 -20 -25 -30

9

10

11

12

8

Frequency (GHz)

11

12

(d)

0

N

0

-5

A

-1

M

-2

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

-4 -5 0

200

ED

-3

400

PT

Reflection loss (dB)

10

25ºC 200ºC 400ºC 600ºC

Frequency (GHz)

Reflection loss (dB)

(c)

9

SC R

Reflection loss (dB)

Reflection loss (dB)

-2

IP T

-5

-1

U

(a)

A

CC E

Temperature (ºC)

600

-10 -15

8.2GHz 9.2GHz 10.2GHz 11.2GHz 12.2GHz

-20 -25 -30 0

200

400

Temperature (ºC)

600

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Fig. 16. Schematic illustration of the microwave absorption mechanisms of the SiCf/SiC-SiCnw composites.

Table captions Table 1 Properties of SiCf/SiC-SiCnw and SiCf/SiC Specimen

Porosity (%) 5.67 7.73

Flexural strength (MPa) 613.8 336.7

Elastic modulus (GPa) 86.8 53.3

A

CC E

PT

ED

M

A

N

U

SC R

IP T

SiCf/SiC-SiCnw SiCf/SiC

Density (g/cm3) 2.21 2.10

Table 2 Summarized RL values, peak frequency, and EAB of the two composites

A

CC E

PT

ED

M

A

N

U

SiCf/SiC-SiCnw

25 200 400 600 25 200 400 600

Peak frequency (GHz) 11.2 10 9.3

EAB (GHz) 0.3 (12.1–12.4) 2.9 (9.5–12.4) 3.8 (8.5–12.3) 2.9 (8.2–11.1)

IP T

SiCf/SiC

Minimum RL values (dB) −1.4 −2.3 −3.5 −4.9 −10.2 −16.5 −26.3 −28.3

SC R

samples

Temperature (°C)

Table 3 comparison of microwave absorption properties of some relevant composites

A

A

CC E

PT

ED

M

* RT represent the abbreviations of room temperature

EAB (GHz)

U

Reference [53] [28] [54] [55] [56] [57] [27]

IP T

3 1.78 2.2 1.8 4.7 3.7 2.1 4.2 4.2 3.4 3.9 1.3 2.8

SC R

Minimum RL values (dB) −32 −18.7 −57 −19 −27.8 −30 −21.5 −30.72 −39.42 −69.3 −50.13 −16.5 −47.5

N

Temperature Optimum samples Range thickness (°C) (mm) * CNTs/SiBCN RT 3 SiCnw/SiOC RT 2.9 SiCnw/Si3N4 RT 2.3 SWCNT/Epoxy RT 9.7 C/SiCnw/Paraffin RT 1.7 SiCnw/Paraffin RT 4.6 Cf/SiCnw/Silicon RT 2 100 4 CNTs/SiO2 500 3.5 RT 2.35 G/SiCnw/SiOC 400 2.3 RT 4.5 SiCf/SiC-SiCnw 600 2.5

[42] [46] This work